Fatigue growth prediction of center slant crack in rectangular plate

This paper presents a numerical prediction model of mixed‐mode crack fatigue growth in a plane elastic plate. It involves a formulations of fatigue growth of multiple crack tips under mixed‐mode loading and a displacement discontinuity method with crack‐tip elements (a boundary element method) proposed recently by Yan is extended to analyse the fatigue growth process of multiple crack tips. Due to an intrinsic feature of the boundary element method, a general growth problem of multiple cracks can be solved in a single‐region formulation. In the numerical simulation, for each increment of crack extension, remeshing of existing boundaries is not necessary. Crack extension is conveniently modelled by adding new boundary elements on the incremental crack extension to the previous crack boundaries. At the same time, the element characters of some related elements are adjusted according to the manner in which the boundary element method is implemented. As an example, the present numerical approach is used to analyse the fatigue growth of a centre slant crack in a rectangular plate. The numerical results illustrate the validation of the numerical prediction model and can reveal the effect of the geometry of the cracked plate on the fatigue growth.     

Fatigue crack growth in a structural steel under single and multiple periodic overload cycles

Preliminary results of a research program on fatigue crack growth in a low-carbon steel under a variable amplitude loading are presented. First, test results are reported on crack growth under simple loading sequences containing single and multiple tensile overloads applied periodically between smaller, constant amplitude cycles. Next, the observed crack growth behaviour is compared to predictions from a theoretical model developed by the authors.     

Fatigue life prediction of interference fit fastener and cold worked holes

The purpose of this study is to present a methodology which estimates the fatigue life of interference fit fastener and cold worked holes. This methodology is mainly based on the determination of local stress with finite-element model computations and on the use of a multiaxial fatigue model. This approach is compared with experimental results carried out on test specimens representative of narrow body civil aircraft components. Analysis of the results shows a highly satisfactory correlation between prediction by calculation and experimental data.     

Fatigue crack nucleation and growth in filled natural rubber

Rubber components subjected to fluctuating loads often fail due to nucleation and the growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This investigation explores the nucleation and growth of cracks in filled natural rubber. Both fatigue macro‐crack nucleation as well as fatigue crack growth experiments were conducted using simple tension and planar tension specimens, respectively. Crack nucleation as well as crack growth life prediction analysis approaches were used to correlate the experimental data. Several aspects of the fatigue process, such as failure mode and the effects of R ratio (minimum strain) on fatigue life, are also discussed. It is shown that a small positive R ratio can have a significant beneficial effect on fatigue life and crack growth rate, particularly at low strain range.     

Fatigue life prediction of offshore tubular joints using fracture mechanics

ABSTRACT Fatigue crack growth calculations were performed on offshore tubular joints using the Paris crack growth law. The stress intensity factors required for such calculations were obtained from T‐butt solutions previously proposed by the authors. The applicability of the solutions to tubular joints was first demonstrated by comparing the fatigue life of a base case with that obtained from a mean S–N curve, and the influence on fatigue life of various factors including load shedding, the size of initial defects, weld geometry, etc. was investigated. The solutions were then used to predict the lives of tubular T‐joints from an experimental database. The results show that the solutions underestimate the fatigue life; this underestimation was shown to be primarily due to ignoring the combined effects of load shedding and the intersection stress distribution. In general, however, the trends in the predicted fatigue lives with joint geometry and other details were seen to be superior to predictions from the S–N approach, with the solutions significantly reducing the dependency on loading mode exhibited by the test data.     

Fatigue life prediction of a structural steel under service loading

Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Führing models erroneously predict retarded crack growth.     

Fatigue crack growth, physical understanding and practical application

The paper presents a discussion on two problems associated with fatigue crack growth in aluminium alloys. First, the application of the similarity approach to crack growth prediction in specimens and structures of aluminium alloys is discussed. The significance of similarity conditions is emphasized and the K-dominated zone is briefly addressed. Secondly, the significance of water vapour for fatigue crack growth in aluminium alloy is reported with a case history of subsurface crack initiation and crack growth in vacuum. Some comments are presented on physical understanding and practical applications.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.     

Sigmoidal crack growth rate curve: statistical modelling and applications

The present paper proposes a statistical model for describing sigmoidal crack growth rate curves. Major novelties are: a) exploitation of the maximum likelihood principle for obtaining material estimates by pooling together experimental data belonging to the different crack propagation regions; b) a general formulation which allows to adopt different sigmoidal models and any kind of statistical distribution for the model variables; c) fatigue life predictions through numerical integration of analytical functions with no need of Monte Carlo simulations. Experimental data taken from NASGRO database are used to check the validity of the statistical model in estimating material parameters included in the crack growth NASGRO algorithm. Illustrative plots of number of cycles to failure and crack length after a given number of cycles are presented, showing good agreement between the proposed statistical model and NASGRO results.     

EIFS-based crack growth fatigue life prediction of pitting-corroded test specimens

Corrosive environment causes corrosion pits at material surface and reduces the fatigue strength significantly. Fatigue crack usually initiates at and propagates from these locations. In this paper, a general methodology for fatigue life prediction for corroded specimens is proposed. The proposed methodology combines an asymptotic stress intensity factor solution and a power law corrosion pit growth function for fatigue life prediction of corroded specimens. First, a previously developed asymptotic interpolation method is proposed to calculate the stress intensity factor (SIF) for the crack at notch roots. Next, a growing semi-circular notch is assumed to exist on the specimen’s surface under corrosive environments. The notch growth rate is different under different corrosion conditions and is assumed to be a power function. Fatigue life can be predicted using the crack growth analysis assuming a crack propagating from the notch root. Plasticity correction is included into the proposed methodology for medium-to-low cycle fatigue analysis. The proposed methodology is validated using experimental fatigue life testing data of aluminum alloys and steels. Very good agreement is observed between experimental observations and model predictions.     

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, d a /d N , as functions of multiple variables, including stress intensity factor range, Δ K , stress ratio, R , crack closure stress intensity factor, K _cl , the maximum stress intensity factor K _max , nominal specimen thickness, t , frequency, Ω , and temperature, T . A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY-80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.     

Fatigue life prediction of cracked padded plates

The fatigue crack propagation analyses of padded plates are conducted to predict the crack growth behaviour under various loading conditions. The fatigue life of a padded plate with a single edge crack originating from the weld toe is calculated using the weight function approach. The fatigue strength of padded plates with different pad thickness under remote loading conditions was investigated and compared to the T-plate joint. The improvement of the fatigue strength of the pad design is verified.
  The thickness effect of the padded plate was investigated using the fracture mechanics approach. The geometrically similar model pairs with different initial crack sizes were investigated under remote loading conditions. It was shown that the thickness effect depends on both stress concentration and initial crack size.     

Fatigue and creep crack growth of Alloy 800 and Alloy 617 at high temperatures

A common evaluation is given for creep crack growth and fatigue crack growth experiments which have been performed at the companies ABB, Siemens-KWU and KFA. The materials under investigation were X10NiCrAlTi32 20 (Alloy 800) and NiCr22Col2Mo (Alloy 617). Several production lots and semi-finished materials as well as welded materials have been tested. Testing techniques differed at the different labs. In order to eliminate the influence of individual testing techniques, material from some production lots was investigated at different labs. The given data cover fatigue crack growth (the materials were tested between room temperature and 1050°C; the influence of temperature, R?ratio, and frequency was investigated) and creep crack growth (Alloy 800 was tested between 550°C and 900°C, Inconel 617 between 800°C and 1000°C; the evaluation was done on the basis of the fracture mechanics parameters K₁ and C*).     

Fatigue crack growth from sharp notches

Notch-like stress raisers occur widely in engineering components. They are preferred sites for crack initiation when the components are subjected to cyclic loadings. Thus the growth of cracks initiated from notches is very relevant to design against fatigue failures. Schematic models proposed to explain the departure of notch crack growth from linear elastic fracture mechanics predictions are briefly reviewed. Different methods of measuring crack closure are compared. It is found that the commonly employed notch-mouth clip-gauge method is not sensitive enough to detect the closure of short cracks in regions of notch plasticity. Various mechanics parameters have been claimed to be able to bring the notch crack and long crack growth rate data to a single base. In the present work on double-edge notched AISI 316 stainless steel specimens, it is found that none of them is able to correlate satisfactory all the experimental data.     

Fatigue crack growth of a welded tube–flange connection under bending and torsional loading

In this contribution the results of an experimental investigation into the fatigue crack growth of welded tube-to-plate specimens of steel StE 460 under bending, torsion, and combined in-phase and out-of-phase bending/torsion loading are presented. The tests were performed at stress ratios of R = −1 and R = 0. The residual stresses were reduced by thermal stress relief. The fatigue crack development is compared with the prediction on the crack growth rates of Paris. Individual stress intensity factors for the semielliptical surface cracks in the tube-flange specimens are approximated on a weight function analogy using the published solutions of other workers.     

Fatigue life prediction of vulcanized natural rubber under proportional and non-proportional loading

To investigate the multiaxial fatigue properties of vulcanized natural rubber (NR), a series of tests including both proportional and non-proportional loading paths on small specimens were performed. The existing fatigue life prediction approaches are evaluated with life data obtained in the tests. It is shown that the equivalent strain approach presents a good prediction of the fatigue life although it has a certain shortcoming. Compared with the strain energy density (SED) model, the cracking energy density (CED) model represents the portion of SED that is available to be released by virtue of crack growth on a given material plane, so it gives better results in the life prediction. Some of the approaches based on critical plane which are widely used for metal fatigue are also tested in this paper, and the results show that the Chen-Xu-Huang (CXH) model gives a better prediction, compared with the Smith-Watson-Topper (SWT) and Wang–Brown (WB) model. A modified Fatemi–Socie's model has also been introduced, and the results show that the modified model can be used to predict the fatigue life of rubber material well.     

Fatigue crack growth with fiber failure in metal-matrix composites

Crack growth during the fatigue of fiber-reinforced metal-matrix composites can be predicted analytically by determining the reduction in the crack tip stress intensity range resulting from fiber bridging. Various canonical functions exist that relate the crack tip stress intensity range to bridged crack geometries and loading for both infinite and finite width specimens; however, comprehensive crack growth predictions incorporating fiber failure require knowledge of the maximum fiber stress in the bridged zone for all notch sizes and crack lengths. Previous modeling efforts have been extended to predict complete growth curves with fiber failure for specimens of finite width. Functions for maximum fiber stresses in the bridged zone are presented here for a center crack in tension and edge cracks in tension and bending. The rapid increase in crack growth when fibers fail emphasizes the importance of determining the loads and notch sizes that mark the beginning of fiber failure. Critical loads for given notch sizes and fiber strengths are easily determined for finite width specimens using the functions presented in this work.